This invention relates in general to the art of extracting a liquid fossil fuel (oil, gas or liquified coal) from beneath the earth's surface and other such underground activities. Subsurface actuators are used in the drilling, testing, completing, and producing phases of oil field activity. The primary application of this invention is to subsurface safety valves for installation in wells that are already producing oil or gas. However, the principles of the invention have other applications as well.
More specifically, the present invention is directed to an arrangement for verifying correct operation or determining intermittent or marginal performance, of a subsurface device, such as a valve, that is controlled by an electromagnetic signal transmitted by a control station located at the earth's surface, such as at a ground station or on a well platform in a sea. Although the invention as described pertains primarily to the verification of valve operation, it applies to any subsurface device actuated or controlled by a signal transmitted from the earth's surface.
Self-contained valve control systems for downhole installation are known. One example can be found in U.S. Pat. No. 3,665,955--Conner, Sr. (May 30, 1972). The subject matter of this U.S. patent is hereby incorporated by reference as if fully set forth herein. The valve can be responsive to pressure, flow rate, sound, or electromagnetic signals at the valve location. Also, wireless signalling to/from a subterranean device is known. An example can be found in U.S. Pat. No. 3,967,201--Rorden (June 29, 1976). The subject matter of this U.S. patent is hereby incorporated by reference as if fully set forth herein.
A significant problem with valves or other actuating devices installed downhole is that it is not possible to know for certain whether a particular installation is working properly. The low frequency communication channel between the earth's surface and the subsurface valve is a noisy one (low S/N ratio). Not all control information transmitted at the surface is properly received and acted upon. Over the course of the life of a particular subsurface installation, the subsurface installed device may be called upon many times to respond to various control signals transmitted at the earth's surface for opening and closing a valve. Later, when the valve or other device is removed from its subsurface installation, it is not known whether the device responded properly to the various signals transmitted to it. In other words, there is no presently known system for verifying that a transmission of control information from the surface to a downhole installation was effective.
Verification of data transmitted from a subsurface installation to the earth's surface was addressed in U.S. Pat. No. 4,216,536--More (August 5, 1980). The disclosure of that patent is incorporated herein by reference as if fully set forth herein. In the More '536 patent, there is described a system for storing downhole data (measurements of various physical parameters at the downhole location) sensed by a subsurface device and transmitting that data to the surface whereat it is received and stored. Later, after retrieving the subsurface device from its downhole installation, the data stored downhole is read from storage and compared with the data received and stored at the surface. Thus, the effectiveness of the transmission of data from the downhole installation to the earth's surface can be determined.
The problem remains as to how to verify correct operation or determine intermittent or marginal performance of a downhole actuating device, such as a valve, in response to control signals transmitted from the earth's surface to the downhole device over a noisy communication channel.